1. Field
Embodiments of the present invention relate to a linear motor, and in particular to a linear motor used in precision drive applications such as semiconductor lithography.
2. Background
Many automated manufacturing processes require the ability to move a workpiece quickly and accurately into a location at which one or more process steps are performed. In some applications, such as semiconductor lithography, such precision positioning must be achieved with an accuracy approaching nanometers, and with speeds consistent with the throughput requirements of modern day lithographic processes.
The challenges associated with positioning equipment to accuracies of the order of nanometers are significant, particularly in the context of photolithography systems. In the photolithography context, substrates undergo multiple processes that result in a modern day integrated circuit. Many of these processes require that multiple steps be performed on a substrate, where each step requires excruciating alignment from one step to a successive step. Many of these steps require the substrate to be moved into and out of one or more stages for patterning and other operations. Not only are nanometer alignments a significant challenge but the throughput of modern day lithographic systems demands rapid movement to and from those precise locations. Moreover, many lithographic systems contain two or more tables such that preparatory steps can be accomplished in parallel with the main processing steps. The use of multiple tables requires expeditious re-positioning of the substrate in order to capitalize on the benefit of the multiple tables.
Linear motors have become a preferred means of positioning in lithography by virtue of their accuracy, acceleration, travel range, packaging size, improved power dissipation, reliability and longevity. In many lithographic applications, arrays of linear motors are used to maximize the actuator force while meeting volume and other requirements of modern lithographic equipment.
A linear motor typically includes of a magnetic circuit having permanent magnets, a back-iron and a coil. When the coil is energized, an electromagnetic interaction between the energized coil and the permanent magnets generates the actuator force used for the precision positioning.
However, some amount of magnetic flux leaks out of the intended magnetic circuit of the linear motor. Because of the proximity of adjacent linear motors, the leakage flux travels through the alternate low resistance path offered by the permanent magnets and ferromagnetic materials within the adjacent linear motors. Such a path for the leakage flux results in undesirable cross-talk forces acting in conflict with the desired forces produced by the linear motors.
Specifically, these cross-talk forces pose the following significant concerns to the use of a linear motor in a precision positioning system. Firstly, a fraction of the nominally available motor force is lost in overcoming cross-talk resistance in the driving direction, which results in increased power dissipation. Secondly, the cross-talk resistance in the driving direction varies with the distance between adjacent linear motors or adjacent ferromagnetic materials. Such variation poses significant challenges to the control system of the linear motor, with potentially unstable consequences. Finally, a cross-talk force component in the lateral direction (i.e., lateral to the driving direction) results in undesired physical forces being applied to the frame to which the linear motors are mounted. The level of such forces can be significant enough to result in deformation to the frame. For example, in a given configuration, a lateral cross-talk force of 0.1 N can result in magnet frame deformations of the order of 20 μm. Such deformations result in significant challenges to linear motor design for position systems that already must account for packaging efficiency, manufacturing tolerances, alignment tolerances and a design safety factor.
Therefore, what is needed is a linear motor that can minimize the impact of magnetic leakage flux, while maintaining the benefits of accurate positioning and rapid acceleration so necessary to meet the modern semiconductor lithography demands.